1. Field of the Invention
This invention relates to dispersants containing lignin sulfonic acid and/or alkali metal, alkaline earth metal, or ammonium salts thereof and to their use in aqueous coal suspensions.
2. Statement of Related Art
In view of the expected shortage of crude oil as a fuel due to the decline in natural reserves worth producing on the one hand and the need to use them as a chemical raw material on the other hand, energy-recovery processes in which aqueous coal suspensions are subjected to high-temperature combustion have recently been gaining in importance. Aqueous coal suspensions offer the major advantage over coal in solid, albeit finely divided, form in that they may be conveyed over relatively long distances at relatively low cost in transport systems known from the transport of crude oil, for example pipelines or overland pipelines. However, the disadvantage is that the dispersion medium, water, which is inert to combustion, reduces the gross calorific value of aqueous coal suspensions. So far as the coal content of aqueous coal suspensions is concerned, an excessive coal content has to be avoided with transportability in mind whereas as high a coal content as possible is desired with the gross calorific value in mind.
It is known from the prior art that aqueous coal suspensions may be provided with additives which improve fluidity during transport in pipelines, but which on the other hand allow an increase in the coal content of the aqueous coal suspensions and hence considerably increase the gross calorific value. Such additives also improve the combustion properties of the aqueous coal suspensions. For example, it is known from German Patent 27 27 943, from EP-B 0 050 412, from EP-A 0 089 766 and from EP-A 0 092 353 that organic dispersants selected from the group comprising organic sulfonic acids and/or alkali metal, alkaline-earth metal or ammonium salts thereof, more especially lignin sulfonic acid and corresponding salts thereof, may be incorporated in aqueous coal suspensions to improve their fluidity and, at the same time, to increase their coal content. Providing the coal used to prepare the coal suspensions has a suitable sieve analysis, it is possible to produce readily pumpable suspensions having coal contents of from 10 to 50% by weight.
However, it is generally desired to further increase the coal content in order to increase the gross calorific value of aqueous coal suspensions, because aqueous coal suspensions can ony be economically used for industrial combustion processes for energy recovery beyond a coal content of 65% by weight coal, based on the total weight of the suspension. In addition, it is desired to further reduce the viscosity of aqueous coal suspensions to provide for convenient transport even through pipelines of comparatively narrow cross-section.